This invention relates to a power supply for a microwave magnetron and to a microwave energy generating system incorporating such power supply and magnetron.
The magnetron is a type of microwave vacuum tube used in conventional microwave ovens as the generator of continuous wave microwave frequency radiation. In turn, the radiation heats or cooks foodstuffs exposed thereto in the oven.
Heretofore, various types of electrical power supplies have been used to convert low line voltages, typically on the order of 110 or 220 volts AC RMS, to the high voltage levels required to supply the microwave magnetron, typically on the order of 3000 or 4000 volts. Such power supplies at present typically include one or more large, heavy and relatively expensive iron core electrical transformers as a component element for the purpose of raising the AC voltage level from the line voltage to the desired high level AC voltage. As a practical matter, power supplies developed for that application employ no more than the minimum number of components necessary to perform the function of providing a predetermined power level to the magnetron consistent with reliability and safety so that the cost of manufacture of the microwave oven is as low in price as possible and preferably within the price range of the ordinary consumer. In view of those cost requirements, the use of relatively sophisticated laboratory type variable power supplies, although suitable and even preferable, technically, for this purpose is economically impractical.
Microwave oven manufacturers presently desire microwave ovens capable of operating at different power levels so that the radiation level may be "tailored" to heat or cook different types of food. Thus in some prior art microwave ovens means have been employed to vary the power level from one level, 600 watts, to another power level, 400 watts, for example by switching a capacitor into or out of the circuit in the high voltage section of the power supply. One drawback to the obvious extension of that principle is that the addition of further capacitors and switches necessary to allow approximation of a continuously variable power level is large. Inasmuch as these electrical components are located wholly or partially in the high voltage section where the voltage range between 3,000 to 4,000 volts they must be capable of functioning at high voltages. Components fulfilling those requirements are very expensive. Other known modifications to existing cooker magnetron power supplies for the magnetron to permit power level adjustment are also known. Thus a device is available for adjusting the magnetron filament current, an operational parameter of the magnetron, which in turn varies the electronic emission of the magnetron and hence its output power level. Other approaches to control the electrical current supplied to the magnetron include Triac, SCR and saturable reactor. All of the foregoing designs, however, retain in their structure the heavy expensive iron core transformer, as they must operate at mains frequency (typically 50 or 60 Hz.). The expensive sophisticated laboratory type power supplies, though suitable, remain economically prohibitive.
Two additional approaches appear in the patent literature as has been made known to me relevant to this subject matter. Recognizing that the core size of a transformer, hence its weight, can be reduced by employing AC of a higher frequency than the 60 Hz. line frequency, U.S. Pat. No. 3,735,237 to Derby describes a specific structure that employs high frequency AC to be applied to the magnetron. The disclosed apparatus appears to increase in component complexity and number, however, and does not provide filament current to the magnetron from the power transformer. A second patent, U.S. Pat. No. 3,862,390 to Noda, describes another type of high-frequency generator which employs high frequency AC for the magnetron and which varies power level. In applicant's opinion, Noda requires a separate filament transformer and perhaps other apparatus for the magnetron, none of which is disclosed, and applicant is otherwise unable to describe the ability or reliability of the disclosed circuit in performance of its intended purpose in actual practice.